This application is based on Application No. 027552 filed in Japan on Feb 2, 2001, Application No. 030245 filed in Japan on Feb 6, 2001, and Application No. 030246 filed in Japan on Feb. 6, 2001, the contents of which are incorporated hereunto by reference.
This invention relates to phosphor materials primarily excited by electron beams accelerated by 1000 V or less, described by the chemical composition formula SrTiO3:Pr, Al, and to the method of manufacture of those phosphor materials.
Formerly, ZnS:Ag phosphor materials, with zinc sulfide as the primary phosphor component, were used in cathode ray tubes (CRTs) as blue phosphors which emitted light when excited by low accelerating voltage electron beams. However, these sulfide phosphors emit sulfide gasses when excited by an electron beam and the phosphor material dissociates and scatters. As a result, problems with oxide filament contamination and reduction in phosphor light emitting efficiency easily develop. Further, these phosphors also have the drawback that red light cannot be emitted.
(ZnCd)S:AgCl phosphor materials have been developed as phosphors which emit light in the red to yellow range with low accelerating voltage electron beams. However, these phosphors not only include cadmium, which can cause environmental contamination, but they also have undesirable light emission characteristics due to poor conductivity. Poor conductivity yields non-uniform light emission and reduced luminance. This is because electrons supplied for excitation cannot flow smoothly and the phosphor becomes negatively charged by the electrons. Specifically, the phosphor becomes charged-up and the negative charge becomes an obstacle to the flow of electrons in the electron beam. In particular, a low accelerating voltage electron beam cannot be smoothly supplied to a negatively charged phosphor, and this causes significant light emission characteristic degradation. To eliminate this drawback, a powder such as In2O3 is mixed into (ZnCd)S:AgCl phosphor to improve conductivity. This phosphor has improved conductivity due to the added powder, but since the conductivity of the phosphor itself is not improved, ineffective current through the conducting powder becomes large. This situation is the cause of reduced light emitting efficiency particularly for low accelerating voltage electron beams.
Phosphor materials with SrTiO3 as their primary component have been developed which do not include cadmium, are applicable with low accelerating voltage electron beams, and emit red light (Japanese Patent Publication HEI 8-85788, 1996). These phosphors have the feature that they do not include the environmental contaminant cadmium, but they do not have desirable light emission characteristics. In particular, they do not have sufficiently long life time. With the objective of eliminating this drawback, phosphors which replace part of the Ti with group IVB elements such as Sn, Si, and Ge have been developed (Japanese Patent Publication HEI 10-273658, 1998). Lifetime characteristics of the SrTiO3:Pr, Al phosphor materials cited in this disclosure can be improved by increasing the amount of Sn added. However, as the amount of added Sn increases, reduction in luminance is a drawback.
The present invention was developed to further resolve these drawbacks. Thus it is the first object of the present invention to provide electron beam excited, light emitting SrTiO3:Pr, Al type phosphor material which can be improved in both luminance and life time characteristics, and to provide its method of manufacture.
The present invention was developed to improve non-uniform light emission caused by lower conductivity associated with phosphors having SrTiO3 as a primary component. Thus it is the second object of the present invention to provide phosphor material which can effectively eliminate light emission non-uniformity with low accelerating voltage electron beams.
The first electron beam excited, light emitting phosphor material of the present invention has the chemical composition formula (Sr1xe2x88x92xxe2x88x92y, Mgx, Cay)TiO3:Pr,Al, where x+y is specified in the range 0.001 to 0.05. If x+y is less than 0.001, luminance improvement effects become insufficient and effects allowing lifetime improvement become small. In contrast, if x+y is made greater than 0.05, luminance drops abruptly. Consequently, considering both luminance and lifetime, x+y is limited to the extremely small range specified above for the phosphor of the present invention.
The phosphor of the present invention has the value of x+y specified within the range described above and with the possibility of x=0 or y=0. A phosphor with x=0 has part of the Sr replaced by Ca only, and a phosphor with y=0 has part of the Sr replaced by Mg only.
The value of x+y in the formula (Sr1xe2x88x92xxe2x88x92y, Mgx, Cay)TiO3:Pr,Al is specified within the range described above for the phosphor of the present invention, and as shown in FIG. 1, lifetime characteristics can be greatly improved while improving luminance characteristics by replacing part of the Sr with trace amounts of Mg and Ca. Further, lifetime characteristics can be greatly improved while improving luminance characteristics by replacing part of the Sr with only Ca for x=0, or by replacing part of the Sr with only Mg for y=0.
The value of x+y in the chemical formula is preferably in the range from 0.003 to 0.02, and this allows marked improvement in lifetime characteristics while further improving luminance characteristics. The phosphor material of the present invention is suitable for phosphor displays with electron beam accelerating voltages of 1000 V or less, or as phosphors used in displays having field emission cathodes as sources of electrons.
The amount of the activating element Pr included is preferably from 0.0001 to 0.1 mole per mole of Sr, and the amount of Al included is preferably from 0.001 to 1.0 mole per mole of Ti. Further, part of the activating element Al may also be replaced by at least one of the elements, Ga and In.
The first phosphor of the present invention has the chemical formula (Sr1xe2x88x92xxe2x88x92y, Mgx, Cay)TiO3:Pr,Al, where x+y is specified in the range 0.001 to 0.05, and by replacing part of the Sr with Mg and Ca, has the feature that lifetime characteristics can be greatly improved while improving luminance characteristics.
The second electron beam excited, light emitting phosphor material of the present invention has the chemical composition formula SrTiO3:Pr,Al, and the surface of phosphor particles are diffused with a diffusing agent containing at least one type of the following elements; Be, Mg, Ca, Sr, and Ba. SrTiO3:Pr,Al phosphors emit red light when excited by low energy electron beam, but the impact energy of a low energy electron beam is small and light emission is from the surface of phosphor particles. By diffusing a diffusing agent containing at least one type of the elements, Be, Mg, Ca, Sr, and Ba, into the surface region of phosphor particles of the present invention, light emission characteristics of the surface of phosphor particles are improved. Consequently, low accelerating voltage, low energy electron beam light emission characteristics of the SrTiO3:Pr,Al phosphor material of the present invention can be improved.
The diffusion depth of the diffusing agent into the phosphor particle interior is preferably in the range of 50 xc3x85 to 400 xc3x85 from the surface. Diffusion depth of the diffusing agent into the phosphor particle can be adjusted by firing time and temperature during the re-firing process. If firing time and temperature during the re-firing process are increased, the diffusing agent will diffuse deeper into the phosphor particle. Diffusion depth is set from 50 xc3x85 to 400 xc3x85 because light emission characteristics under low accelerating voltage, low energy electron beam excitation decrease when diffusing agent diffusion depth exceeds 500 xc3x85. Consequently, the diffusion depth of the diffusing agent into the surface region of the phosphor particle is preferably less than 400 xc3x85. Therefore, since this phosphor material has diffusing agent diffused within a range from the surface to 400 xc3x85, light emission characteristics can be improved when excitation is by low energy electron beam.
Phosphor particles are re-fired with diffusing agent in contact with particle surfaces for diffusion into phosphor particle surface regions. Phosphor material re-fired to diffuse diffusing agent into surface regions is preferably made to include 0.001 to 15 weight % diffusing agent. Firing temperature in the re-firing process is 400xc2x0 C. to 1300xc2x0 C. Phosphor material re-fired at this temperature has diffusing agent diffused and incorporated into phosphor particle surface regions.
The second, SrTiO3:Pr,Al, phosphor material of the present invention is preferably made by re-firing at 400xc2x0 C. to 1300xc2x0 C. SrTiO3:Pr,Al phosphor material re-fired at this temperature, with diffusing agent covering or attached to phosphor particle surfaces during re-firing, has diffusing agent diffused from the phosphor particle crystalline surface towards the particle interior.
The second, SrTiO3:Pr,Al, phosphor material of the present invention is suitable for phosphor displays with electron beam accelerating voltages of 1000 V or less, or as phosphors used in displays having field emission cathodes as sources of electrons.
The method of manufacture of the second phosphor material of the present invention comprises a first firing process to fire raw materials to form SrTiO3:Pr,Al phosphor, and a re-firing process to again fire the SrTiO3:Pr,Al phosphor in contact with a diffusing agent including at least one type of the elements, Be, Mg, Ca, Sr, and Ba. In the re-firing process, an amount of diffusing agent is added to include 0.001 to 15 weight % diffusing agent in the re-fired phosphor. The amount of diffusing agent added affects the light emission-characteristics of the phosphor. If too little diffusing agent is added, any effect of the diffusing agent to improve light emission characteristics cannot be expected. On the other hand, if too much diffusing agent is added, luminance will drop. Turning to FIG. 2, luminance and luminance maintenance are shown as a function of Ca content, where Ca is diffused into phosphor particles as a diffusing agent. As shown in FIG. 2, luminance and luminance maintenance show improvement as the amount of diffusing agent increases. At a diffusing agent content of 0.1 weight %, luminance reaches a maximum. If the amount of diffusing agent is further increased, luminance gradually decreases.
Firing temperature during the re-firing process is, for example, 400xc2x0 C. to 1400xc2x0 C., preferably, 500xc2x0 C. to 1300xc2x0 C., and more preferably, 800xc2x0 C. to 1250xc2x0 C. If temperature during the re-firing process is too low, diffusing agent added to the phosphor for re-firing cannot diffuse sufficiently into the interior of phosphor particles and little light emission characteristic improvement will be gained. In contrast, if temperature during the re-firing process is too high, diffusing agent will diffuse well into phosphor particle interiors and the effect of improving light emission characteristics near particle surfaces will decrease.
Since the method of manufacture of the second phosphor material of the present invention diffuses at least one type of diffusing agent, Be, Mg, Ca, Sr, and Ba, into the surface region of phosphor material having the chemical formula SrTiO3:Pr,Al, it has the feature that phosphor particle surface light emission characteristics can be improved. In particular, the SrTiO3:Pr,Al phosphor of the present invention can be excited by a low accelerating voltage, low energy electron beam to emit red light, and its luminance and luminance maintenance can be improved.
Further, since the method of manufacture of the second phosphor material of the present invention comprises a first firing process to fire raw materials into SrTiO3:Pr,Al phosphor, and a re-firing process to again fire the phosphor in contact with diffusing agent, it has the feature that diffusing agent can be diffused into phosphor particle surface regions by simple process steps allowing both luminance and lifetime characteristics to be improved in an ideal fashion.
Finally, the third electron beam phosphor material of the present invention is a phosphor with the chemical formula SrTiO3:Pr,Al characterized by a Sr/Ti molar ratio of 0.88 to 0.99. By specifying the Sr/Ti molar ratio within this range, light emission non-uniformity can be effectively eliminated while minimizing luminance reduction.
FIG. 3 shows light emission non-uniformity as a function of Sr/Ti molar ratio, and FIG. 4 shows relative luminance as a function of Sr/Ti molar ratio. Here, light emission non-uniformity was measured as follows.
100 parts by weight of a mixture of phosphor (90%) and In2O3 conductive material (10%) were mixed with 90 parts by weight of vehicle containing 2% organic binder. Phosphor screens were fabricated by applying this mixture to a substrate using printing methods to make circular phosphor spots 25 xcexcm thick and 5 mm in diameter. Electrons with an accelerating voltage of 12 V and current flow of 0.6 mA were directed at the phosphor screens for light emission to visibly observe non-uniformity. For each phosphor, five phosphor spots were made, and non-uniformity measurement was performed by recording xe2x80x9ca non-uniformityxe2x80x9d if non-uniform emission occurred at even one location at the measured spot. Non-uniformity generation rate was computed by dividing the number of non-uniform locations by the total number of measurements.
From these figures, it is clear that luminance drops at Sr/Ti molar ratios below 0.88. Further, light emission non-uniformity increases when Sr/Ti molar ratios are greater than 0.99. To further reduce luminance decrease, the Sr/Ti molar ratio is made 0.92 to 0.99.
The third phosphor material of the present invention is suitable for phosphor displays with electron beam accelerating voltages of 1000 V or less, or in display device which use field emission cathodes as sources of electrons. Further, since light emission non-uniformity is eliminated by a specified Sr/Ti molar ratio for the SrTiO3:Pr,Al phosphor described above, it can be used without adding conductive material such as In2O3 to improve conduction.
In the chemical formula SrTiO3:Pr,Al, the amount of activating agent, Pr, included is 0.0001 to 0.1 mole per mole of Sr. The amount of activating agent, Al, included is 0.001 to 1.0 mole per mole of Ti. Specification of activating agent content within these ranges is for the purpose of making phosphor luminance as great as possible, and activating agent content exceeding these ranges results in a drop in luminance.
The third SrTiO3:Pr,Al phosphor of the present invention has the feature that light emission non-uniformity can be drastically reduced while minimizing luminance drop by confining the Sr/Ti molar ratio to a specified range. In particular, by specifying the Sr/Ti molar ratio in the range 0.92 to 0.99, the phosphor material has the feature that light emission non-uniformity can be made extremely small while maintaining luminance within 10% below the luminance of a phosphor with a Sr/Ti molar ratio of 1.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.